U.S. patent number 7,280,264 [Application Number 10/569,097] was granted by the patent office on 2007-10-09 for magneto-optical device.
This patent grant is currently assigned to FDK Corporation. Invention is credited to Yuji Goto, Masaharu Hoshikawa, Takashi Kato, Mikio Kitaoka, Akitoshi Mesaki, Chiharu Nishida, Tsugio Tokumasu, Hiromitsu Umezawa, Toshihiko Watanabe.
United States Patent |
7,280,264 |
Goto , et al. |
October 9, 2007 |
Magneto-optical device
Abstract
It is possible to reduce the size of a magneto-optical device,
increase the speed of optical control, simplify power supply
structure and its control, and maintain a Faraday rotation angle in
an arbitrary state even after shut-off of the excitation current.
The magneto-optical device includes a magnetic yoke (10) made of a
high-permeability magnetic material, the magnetic yoke including a
tabular portion (16) and four pillar portions (18) protruding from
one side of the tabular portion (16), a coil (12) wound on each of
the pillar portions, and a magneto-optical element (14) arranged in
an open-magnetic-circuit region surrounded by the end portions of
the four pillar portions. A magnetic field obtained by a coil is
applied to the magneto-optical element.
Inventors: |
Goto; Yuji (Minato-ku,
JP), Kitaoka; Mikio (Minato-ku, JP),
Umezawa; Hiromitsu (Minato-ku, JP), Tokumasu;
Tsugio (Minato-ku, JP), Watanabe; Toshihiko
(Minato-ku, JP), Mesaki; Akitoshi (Minato-ku,
JP), Kato; Takashi (Minato-ku, JP),
Hoshikawa; Masaharu (Minato-ku, JP), Nishida;
Chiharu (Minato-ku, JP) |
Assignee: |
FDK Corporation (Tokyo,
JP)
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Family
ID: |
34277646 |
Appl.
No.: |
10/569,097 |
Filed: |
August 26, 2004 |
PCT
Filed: |
August 26, 2004 |
PCT No.: |
PCT/JP2004/012279 |
371(c)(1),(2),(4) Date: |
February 22, 2006 |
PCT
Pub. No.: |
WO2005/022243 |
PCT
Pub. Date: |
March 10, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070002425 A1 |
Jan 4, 2007 |
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Foreign Application Priority Data
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Aug 28, 2003 [JP] |
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2003-305011 |
Feb 19, 2004 [JP] |
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2004-043552 |
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Current U.S.
Class: |
359/283;
359/281 |
Current CPC
Class: |
G02F
1/09 (20130101); G02F 1/093 (20130101) |
Current International
Class: |
G02F
1/09 (20060101) |
Field of
Search: |
;359/280-284 |
References Cited
[Referenced By]
U.S. Patent Documents
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6854125 |
February 2005 |
Mizuno et al. |
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Foreign Patent Documents
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62-95515 |
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May 1987 |
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JP |
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9-33871 |
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Feb 1997 |
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JP |
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Other References
Akitoshi Mesaki et al., "Development of Fast Response Faraday
Rotator", Proceedings of the IEICE Conference, Mar. 8, 2004, vol.
2004, Electronics 1, p. 280. cited by other .
Chiharu Nishida et al., "Development of Fast Response VOA",
Proceedings of the IEICE Conference, Mar. 8, 2004, vol. 2004,
Electronics 1, p. 283. cited by other.
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Primary Examiner: Mack; Ricky
Assistant Examiner: Choi; William C
Attorney, Agent or Firm: Wenderoth, Lind & Ponack,
L.L.P.
Claims
The invention claimed is:
1. A magneto-optical device comprising: a magnetic yoke made of a
high-magnetic-permeability material, the magnetic yoke including a
tabular portion, and at least three pillar portions protruding
perpendicularly from one side of the tabular portion; coils wound
around the pillar portions; and a magneto-optical element arranged
in an open-magnetic-circuit space surrounded by the respective
top-end portions of the pillar portions, wherein magnetic fields
generated through the coils are applied to the magneto-optical
element.
2. The magneto-optical device according to claim 1, wherein the
magnetic yoke has a structure in which an approximately
quadrangular tabular portion and four quadrangular pillar portions
protruding from the vicinities of the four corners of the tabular
portion, perpendicularly and in the same direction, are
continuously integrated.
3. The magneto-optical device according to claim 1, wherein as the
high-magnetic-permeability material, ferrite is utilized, and for
the magneto-optical element, a bismuth-substituted rare-earth
iron-garnet single crystal is utilized.
4. The magneto-optical device according to claim 1, wherein, by
controlling respective directions and/or values of electric
currents supplied to the coils, the magnetization direction of the
magneto-optical element can be changed.
5. The magneto-optical device according to claim 1, wherein the
magnetic yoke has a structure having 2n (where n.gtoreq.2) pillar
portions protruding perpendicularly from one side of the tabular
portion; the polarities of magnetic fields applied to the
magneto-optical element, through the coils diagonally opposing each
other with respect to the magneto-optical element, are reverse to
each other; and a pair of the coils that are diagonally opposing
each other with respect to the magneto-optical element and
connected in parallel or in series is driven by a common power
supply unit.
6. The magneto-optical device according to claim 1, wherein the
tabular portion of the magnetic yoke is formed of a semi-hard
magnetic material, and the pillar portion is formed of a
soft-magnetic material.
7. A variable optical attenuator for, with light polarizers
arranged before and after a magneto-optical device in the light
path thereof, controlling an attenuation value of output optical
power versus input optical power, the magneto-optical device
comprising: a magnetic yoke made of a high-magnetic-permeability
material, the magnetic yoke including a tabular portion, and at
least three pillar portions protruding perpendicularly from one
side of the tabular portion; coils wound around the pillar
portions; and a magneto-optical element arranged in an
open-magnetic-circuit space surrounded by the respective top-end
portions of the pillar portions, wherein magnetic fields generated
through the coils are applied to the magneto-optical element, and
the magnetization direction of the magneto-optical element can be
changed, by controlling respective directions and/or values of
electric currents supplied to the coils.
8. An optical switch for controlling switchedly output light versus
input light, the optical switch having a configuration in which
light polarizers are arranged before and after a magneto-optical
device in the light path thereof, and
residual-magnetization-natured bismuth-substituted rare-earth iron
garnet single crystal is utilized as a magneto-optical element, the
magneto-optical device comprising: a magnetic yoke made of a
high-magnetic-permeability material, the magnetic yoke including a
tabular portion, and at least three pillar portions protruding
perpendicularly from one side of the tabular portion; coils wound
around the pillar portions; and a magneto-optical element arranged
in an open-magnetic-circuit space surrounded by the respective
top-end portions of the pillar portions, wherein a magnetic field
generated through the coils is applied to the magneto-optical
element, and the magnetization direction of the magneto-optical
element can be changed, by controlling respective directions and/or
values of electric currents supplied to the coils.
9. A magneto-optical device array in which magneto-optical devices
are arranged side by side, the magneto-optical device comprising: a
magnetic yoke made of a high-magnetic-permeability material, the
magnetic yoke including a tabular portion and at least three pillar
portions protruding perpendicularly from one side of the tabular
portion; coils wound around the pillar portions; and a
magneto-optical element arranged in an open-magnetic-circuit space
surrounded by the respective top-end portions of the pillar
portions, wherein magnetic fields generated through the coils are
applied to the magneto-optical element.
10. A magneto-optical device comprising: a magnetic yoke made of a
high-magnetic-permeability material, the magnetic yoke including 2n
(where n.gtoreq.2) pillar portions protruding perpendicularly from
one side of a tabular portion; coils wound around the pillar
portions; and a magneto-optical element arranged in an
open-magnetic-circuit space surrounded by the respective top-end
portions of the pillar portions, wherein: magnetic fields generated
through the coils are applied to the magneto-optical element; the
polarities of magnetic fields applied to the magneto-optical
element, through the coils diagonally opposing each other with
respect to the magneto-optical element, are reverse to each other;
and a pair of the coils that are diagonally opposing each other
with respect to the magneto-optical element and connected in
parallel or in series is driven by a common power supply unit.
11. The magneto-optical device according to claim 10, wherein the
magnetic yoke has a structure having an approximately square
tabular portion and four pillar portions protruding from the
vicinities of the four corners of the tabular portion,
perpendicularly and in the same direction.
12. The magneto-optical device according to claim 10, wherein the
magnetic yoke has a tabular portion and pillar portions protruding
in the same direction from one side of the tabular portion, and one
pair of the pillar portions opposing each other with respect to the
magneto-optical element is arranged perpendicular to the optical
axis of the magneto-optical element and the other pair of the
pillar portions opposing each other with respect to the
magneto-optical element is arranged at a specific angle (smaller
than .+-.90.degree.) from the optical axis.
13. The magneto-optical device according to claim 12, wherein the
angle .theta..sub.h between the direction of a first magnetic field
formed through the one pair of pillar portions and the direction of
a second magnetic field formed through the other pair of pillar
portions is given by the following equation:
.theta..sub.h=sin.sup.-1(.theta..sub.fMAX/.theta..sub.f0) where
.theta..sub.f0 is the Faraday rotation angle of the magneto-optical
element in the case where the direction of a saturation combined
magnetic field is parallel to the optical axis, and
.theta..sub.fMAX is a maximal Faraday rotation angle of the
magneto-optical element in the case of actual use.
14. The magneto-optical device according to claim 10, wherein, in
the case where the direction of a saturation combined magnetic
field is parallel to the optical axis of the magneto-optical
element, the Faraday rotation angle of the magneto-optical element
is set to 127.3.degree. or larger, and the coils are driven by
monopolar power supply units.
15. A magneto-optical device comprising: a magnetic yoke having a
tabular portion made of a semi-hard magnetic material and at least
three pillar portions, protruding from one side of the tabular
portion, that are made of a high-magnetic-permeability material;
coils wound around the pillar portions; and a magneto-optical
element arranged in an open-magnetic-circuit space surrounded by
the respective top-end portions of the pillar portions, wherein
magnetic fields generated through the coils are applied to the
magneto-optical element.
16. A magneto-optical device comprising: a magnetic yoke having a
tabular portion made of a high-magnetic-permeability material and
at least three pillar portions, protruding from one side of the
tabular portion, that are made of a semi-hard magnetic material;
coils wound around the pillar portions; and a magneto-optical
element arranged in an open-magnetic-circuit space surrounded by
the respective top-end portions of the pillar portions, wherein
magnetic fields generated through the coils are applied to the
magneto-optical element.
17. A self-latching variable optical attenuator for, with a light
polarizer, a magneto-optical device, and an analyzer arranged in
that order, controlling an attenuation value of output optical
power versus input optical power, the magneto-optical device
comprising: a magnetic yoke having a tabular portion made of a
semi-hard magnetic material and at least three pillar portions,
protruding from one side of the tabular portion, that are made of a
high-magnetic-permeability material; coils wound around the pillar
portions; and a magneto-optical element arranged in an
open-magnetic-circuit space surrounded by the respective top-end
portions of the pillar portions, wherein magnetic fields generated
through the coils are applied to the magneto-optical element.
18. A self-latching variable optical splitter, comprised of a
magneto-optical device, a light polarizer, an analyzer, and a
waveplate, for controlling optical separation ratio through the
magneto-optical device, the magneto-optical device comprising: a
magnetic yoke having a tabular portion made of a semi-hard magnetic
material and at least three pillar portions, protruding from one
side of the tabular portion, that are made of a
high-magnetic-permeability material; coils wound around the pillar
portions; and a magneto-optical element arranged in an
open-magnetic-circuit space surrounded by the respective top-end
portions of the pillar portions, wherein magnetic fields generated
through the coils are applied to the magneto-optical element.
19. A magneto-optical device comprising: a magnetic yoke having a
block-shaped base portion made of a semi-hard magnetic material and
at least four pillar portions, made of a high-magnetic-permeability
material, extending from the base portion to the vicinity of a
space to be an open-magnetic-circuit space; coils wound around the
pillar portions; and a magneto-optical element arranged in the
open-magnetic-circuit space surrounded by the respective top-end
portions of the pillar portions, wherein magnetic fields generated
through the coils are applied to the magneto-optical element, in
three or more directions.
Description
TECHNICAL FIELD
The present invention relates to a magneto-optical device utilized
in a variable optical attenuator, an optical switch, or the like,
and particularly to a magneto-optical device in which a magnetic
yoke having at least three pillar portions protruding from a
tabular portion is utilized, and the magnetization direction of a
magneto-optical element is controlled by a magnetic field generated
through coils wound around the columnar portions.
BACKGROUND ART
In an optical communication system, an optical measurement system,
and the like, a variable optical attenuator is incorporated that is
a device for variably controlling the transmitted optical power.
The device is comprised of a magneto-optical element having the
Faraday effect, a permanent magnet that applies a fixed magnetic
field to the magneto-optical element, and an electromagnet that
applies a variable magnetic field to the magneto-optical element.
Typically, the electromagnet is of a structure in which a coil is
wound around a C-shaped (a shape of a circle having an open
portion) magnetic yoke. By inserting the magneto-optical element
into the open portion of the C-shaped magnetic yoke and applying
electric current to the coil, a desired variable magnetic field is
applied to the magneto-optical element.
The direction of a constant magnetic field H.sub.r generated by the
permanent magnet is made approximately parallel with the optical
axis of the magneto-optical element, the magneto-optical element is
magnetically saturated, and the maximal Faraday rotation in an
actual use is caused. In other words, the Faraday rotation angle
.theta..sub.f0 in the Faraday arrangement of the permanent magnet
and the magneto-optical element and the maximal Faraday rotation
angle .theta..sub.fMax in an actual use are made to be equal
(.theta..sub.f0=.theta..sub.fMax). Next, a magnetic field H.sub.v
whose direction is approximately perpendicular to the direction of
the magnetic field H.sub.r generated by the permanent magnet is
generated by the electromagnet, the magneto-optical element is
arranged in the combined magnetic field formed of the magnetic
field of the permanent magnet and the magnetic field of the
electromagnet, and the magnitude of the magnetic field generated by
the electromagnet is varied in accordance with the magnitude of the
current that flows in the coil of the electromagnet, whereby the
direction of the combined magnetic field is controlled. The
polarization direction can be controlled in accordance with the
magnitude of the optical-axis-direction component of the combined
magnetic field. The direction .theta..sub.c of the combined
magnetic field H.sub.c is given by the following equation:
.theta..sub.c=tan.sup.-1(H.sub.v/H.sub.r) Variation of the variable
magnetic field H.sub.v varies the direction .theta..sub.c of the
combined magnetic field H.sub.c. The Faraday rotation angle
.theta..sub.f is in accordance with the optical-axis-direction
component of the combined magnetic field H.sub.c, and thus given by
the following equation; therefore, .theta..sub.f can be controlled
by varying .theta..sub.c. .theta..sub.f=.theta..sub.f0.times.cos
.theta..sub.c In other words, by controlling the current that flows
in the coil of the electromagnet for generating H.sub.v,
.theta..sub.f can be controlled.
The Faraday rotation .theta..sub.f0 is caused through the fixed
magnetic field H.sub.r generated by the permanent magnet, and the
Faraday rotation .theta..sub.f is obtained in accordance with the
direction .theta..sub.c of the combined magnetic field H.sub.c
formed of the variable magnetic field H.sub.v generated by the
electromagnet and fixed magnetic field H.sub.r generated by the
permanent magnet. Because, being a magnetic field generated by a
permanent magnet, H.sub.r is constant and not enabled to be zero,
large H.sub.v is required to obtain large .theta..sub.c. In order
to obtain large H.sub.v, it is necessary to increase the number of
windings of the electromagnet coil or to increase current to be
applied to the coil; therefore, the size of the electromagnet is
enlarged or the driving voltage is increased. Moreover, there has
been a problem in that it takes a long time until the change of the
direction .theta..sub.c of the combined magnetic field after the
driving voltage has been changed, i.e., operating speed is low.
The case of a variable optical attenuator has a structure in which
a first light polarizer, a magneto-optical element, and a second
light polarize are arranged in that order, along the optical axis,
a saturation magnetic field is applied through a permanent magnet
to the magneto-optical element, and a variable magnetic field whose
direction is different from that of the saturation magnetic field
is applied through an electromagnet to the magneto-optical element.
Through the permanent magnet and the electromagnet, external
magnetic fields are applied in two or more directions to the
magneto-optical element, and the direction of magnetization of the
magneto-optical element is changed by changing the vector of the
combined magnetic field produced by the permanent magnet and the
electromagnet, whereby the Faraday rotation angle of light that
passes through the magneto-optical element is controlled. For
example, Japanese Patent Laid-Open No. 9-061770 discloses a
magneto-optical device, as described above, in which a
configuration is employed where block-shaped permanent magnets are
arranged above and below the light path, as a means for applying a
fixed magnetic field. Additionally, there is a configuration in
which permanent magnets of ring-shape or the like are arranged
along the optical axis, and a fixed magnetic field is applied in
parallel with the optical axis.
As described above, since, in a conventional variable optical
attenuator, a permanent magnet is utilized to apply a saturation
magnetic field, the magneto-optical element is magnetized in a
constant direction by the permanent magnet, even when no electric
current is supplied to the electromagnet. Accordingly, the size of
a magnetic yoke for an electromagnet utilized for the control of
magnetization is rendered large, or the driving voltage is made
large; therefore, it is difficult to downsize and speed up the
variable optical attenuator.
Moreover, because, when no electric current is applied to the coil
of the electromagnet, the variable magnetic field generated by the
electromagnet becomes substantially zero, the combined magnetic
field to be applied to the magneto-optical element consists only of
the component generated by the permanent magnet, whereby the
Faraday rotation angle returns to the initial condition.
In contrast, in a Faraday rotator utilized as a self-latching
optical switch or the like, the Faraday rotation angle does not
returns to the initial condition, even after the exciting current
for the coil has been cut off; therefore, the condition in the case
where the electric current is applied can be maintained. A Faraday
rotator having the self-latching function is comprised of a
magneto-optical element having the Faraday effect and a
magnetic-field applying device that applies a magnetic field to the
magneto-optical element; normally, no permanent magnet is utilized,
and the magnetic-field applying device consists only of an
electromagnet. As the electromagnet, for example, as disclosed in
Japanese Patent Laid-Open No. 8-211347, a structure is utilized in
which a coil is wound around a C-shaped magnetic yoke. By inserting
the magneto-optical element into the open portion of the C-shaped
magnetic yoke and supplying electric current to the coil, a
magnetic field is applied to the magneto-optical element.
Typically, the control is implemented, with the absolute value of
the electric current kept constant, through polarity-reverse action
of the electric current, accordingly, the number of possible
directions for a magnetic field applied to the magneto-optical
element is only two, i.e., the positive and negative directions
along a single line; therefore, the number of possible states for
the Faraday rotation angle is limited to two states.
In addition, in the Faraday rotator, at least one of the magnetic
yoke and the magneto-optical element is formed of a semi-hard
magnetic material; both of them are magnetized by exciting current;
and after the exciting current is cut off, magnetization remains in
the semi-hard magnetic material. In the case where the
magneto-optical element is formed of a semi-hard magnetic material,
magnetization remains in the magneto-optical element itself;
however, in the case where the magnetic yoke is formed of a
semi-hard magnetic material, a magnetic field generated by residual
magnetization of the magnetic yoke is applied to the
magneto-optical element. In both cases, the residual magnetic-field
vector and the magnetic-field vector in the case where the electric
current is applied are different in magnitude, but the same in
direction. Accordingly, even after the exciting current has been
cut off, the Faraday rotation angle can be maintained in the same
condition as that in the case where the exciting current is
flowing; however, maintainable are only two conditions that are
possible when the electric current is applied, whereby arbitrary
condition cannot be maintained.
As described above, in a conventional variable optical attenuator,
by controlling the exciting current supplied to the coil of the
electromagnet, the magnetization direction of the magneto-optical
element is arbitrarily changed, and, in response to the change of
the magnetization direction, the Faraday rotation angle can
arbitrarily adjusted; however, the Faraday rotation angle cannot be
maintained after the exciting current has been cut off. In
contrast, in a conventional Faraday rotator having a self-latching
function, even after the exciting current has been cut off, the
magnetization direction of the magneto-optical element and the
Faraday rotation angle can be maintained; however, the number of
maintainable conditions is limited to two.
DISCLOSURE OF THE INVENTION
The first issue to be solved by the present invention is that, in
the case of a conventional magneto-optical device in which a
permanent magnet and a electromagnet are combined, because a
magnetic field generated by the permanent magnet always acts, it is
necessary to increase the number of windings of a coil that forms a
variable magnetic field or to increase an electric current to be
supplied, whereby downsizing is impossible, and it is difficult to
speed up the control of light. The second issue to be solved by the
present invention is that, even though the permanent magnet is
simply replaced by the electromagnet, the complexity of the
configuration of the power supply units is raised, and the control
is rendered difficult. The third issue to be solved by the present
invention is that, in the case of a conventional magneto-optical
device in which a permanent magnet and a electromagnet are
combined, the Faraday rotation angle cannot be maintained after the
exciting current has been cut off, and in the case of a
conventional magneto-optical device having a self-latching
function, the number of conditions in which the magnetization
direction of the magneto-optical element and the Faraday rotation
angle can be maintained after the exciting current has been cut off
is not arbitrary but limited to two.
[The First Aspect of the Present Invention]
According to the first aspect of the present invention, there is
provided a magneto-optical device comprising: a magnetic yoke made
of a high-magnetic-permeability material, the magnetic yoke
including a tabular portion, and at least three pillar portions
protruding perpendicularly from one side of the tabular portion;
coils wound around the pillar portions; and a magneto-optical
element arranged in an open-magnetic-circuit space surrounded by
the respective top-end portions of the pillar portions, wherein
magnetic fields generated through the coils are applied to the
magneto-optical element.
The simplest magnetic yoke has a structure in which an
approximately quadrangular tabular portion and four quadrangular
pillar portions protruding from the vicinities of the four corners
of the tabular portion, perpendicularly and in the same direction,
are continuously integrated. In this case, it is preferable to
employ a square tabular portion as well as a square pillar
portion.
For the foregoing members, it is preferable that, as the
high-magnetic-permeability material, ferrite (e.g., Ni--Zn system
ferrite) is utilized, and for the magneto-optical element, a
bismuth-substituted rare-earth iron-garnet single crystal is
utilized. Besides, by controlling respective directions and/or
values of electric currents supplied to the coils, the
magnetization direction of the magneto-optical element can be
changed.
By arranging light polarizers before and after the foregoing
magneto-optical device in the light path thereof and controlling an
attenuation value of output optical power versus input optical
power, a variable optical attenuator can be configured.
Additionally, by arranging light polarizers before and after the
magneto-optical device in the light path thereof and controlling
switchedly output light versus input light, an optical switch can
be configured.
By arranging side by side a plurality of the foregoing
magneto-optical devices, various kinds of magneto-optical device
arrays can be configured.
The magneto-optical device according to the first aspect of the
present invention can be utilized in a variable optical attenuator
or an optical switch. When the magneto-optical device is utilized
in a variable optical attenuator, fixed magnetic field of a
permanent magnet is not necessary, and by generating variable
magnetic fields by utilizing electromagnets only, the magnetization
direction can instantaneously be changed, whereby downsizing and
speedup can be implemented. In addition, since the magnetization
direction can arbitrarily be controlled, the range from -45.degree.
to +45.degree., instead of the range from 0.degree. to 90.degree.,
can be utilized so that the required 90-degree variable amount in
the Faraday rotation angle is obtained; therefore, also in that
sense, the downsizing of components to be utilized can be achieved.
When the magneto-optical device is utilized in an optical switch,
by utilizing a magneto-optical element having a self-latching
function, it is not required to utilize a semi-hard magnetic
material as a magnetic-yoke material; therefore, the magnetic field
generated through the coils and the magnetic yoke may be a critical
mass for reversing the magnetization of the magneto-optical
element, whereby it is possible to achieve downsizing, speedup, and
reduction of power dissipation. Moreover, because the
magnetic-optical device has a structure in which a leakage magnetic
field is weak and magnetic fields converge only on the
magneto-optical element, they do not interfere with one another
even though a plurality of the magneto-optical devices are arranged
side by side; therefore, an array configuration can readily be
realized.
[The Second Aspect of the Present Invention]
According to the second aspect of the present invention, there is
provided a magneto-optical device comprising: a magnetic yoke made
of a high-magnetic-permeability material, the magnetic yoke
including 2n (where n.gtoreq.2) pillar portions protruding
perpendicularly from one side of a tabular portion; coils wound
around the pillar portions; and a magneto-optical element arranged
in an open-magnetic-circuit space surrounded by the respective
top-end portions of the pillar portions, wherein: magnetic fields
generated through the coils are applied to the magneto-optical
element; the polarities of magnetic fields applied to the
magneto-optical element, through the coils diagonally opposing each
other with respect to the magneto-optical element, are reverse to
each other; and a pair of the coils that are diagonally opposing
each other with respect to the magneto-optical element and
connected in parallel or in series is driven by a common power
supply unit. In addition, the power supply unit may be a variable
voltage source or a variable current source.
The most simple magnetic yoke has a structure having an
approximately square tabular portion and four pillar portions
protruding perpendicularly and in the same direction, from the
vicinities of the four corners of the tabular portion. In this
situation, the tabular portion and the pillar portions maybe in an
integrated structure, or a structure may be employed in which each
pillar portion is inserted into holes provided in the tabular
portion and fixed therein.
A structure may be employed in which a magnetic yoke has a tabular
portion and pillar portions protruding in the same direction from
the one side of the tabular portion, one pair of the pillar
portions opposing each other with respect to the magneto-optical
element is arranged perpendicular to the optical axis of the
magneto-optical element and the other pair of the pillar portions
opposing each other with respect to the magneto-optical element is
arranged at a specific angle (smaller than .+-.90.degree.) from the
optical axis. In the case of the foregoing structure, it is
preferable that the angle .theta..sub.h between the direction of a
first magnetic field formed through the one pair of pillar portions
and the direction of a second magnetic field formed through the
other pair of pillar portions is set to the angle given by the
following equation:
.theta..sub.h=sin.sup.-1(.theta..sub.fMAX/.theta..sub.f0) where
.theta..sub.f0 is the Faraday rotation angle of the magneto-optical
element in the case where the direction of a saturation combined
magnetic field is parallel to the light path, and .theta..sub.fMAX
is a maximal Faraday rotation angle of the magneto-optical element
in the case of actual use.
Further, according to the second aspect of the present invention,
there is provided a magneto-optical device in which two magnetic
yokes made of a high-magnetic-permeability material and having 2n
(where n.gtoreq.2) pillar portions protruding perpendicularly from
one side of a tabular portion are combined in such a way that, with
respective coils wound around the pillar portions, the foremost
surfaces of the pillar portions of one magnetic yoke butt against
the foremost surfaces of the corresponding pillar portions of the
other magnetic yoke, a magneto-optical element is arranged in an
open-magnetic-circuit space surrounded by the respective top-end
portions of the pillar portions, and magnetic fields generated
through the coils being applied to the magneto-optical element,
wherein: the coils wound around the corresponding pillar portions
generate magnetic fields that are reverse to each other; the
polarities of magnetic fields applied to the magneto-optical
element, through the coils opposing each other with respect to the
magneto-optical element, are reverse to each other; the magnetic
field generated through the one magnetic yoke and the magnetic
field generated through the other magnetic yoke cooperate to act on
the magneto-optical element; and a pair of the coils diagonally
opposing each other with respect to the magneto-optical element
configure a set and are driven by a common power supply unit.
Moreover, according to the second aspect of the present invention,
there is provided a magneto-optical device comprising: a magnetic
yoke made of a high-magnetic-permeability material, the magnetic
yoke including tabular portions opposing and spaced apart from each
other and 2n (where n.gtoreq.2) pillar portions arranged between
the tabular portions; a plurality of coils wound around each of the
pillar portions; and a magneto-optical element arranged in an
open-magnetic-circuit space surrounded by the respective middle
portions of the pillar portions, wherein: magnetic fields generated
through the coils are applied to the magneto-optical element; the
coils wound around the same pillar portion generate magnetic fields
that are reverse to each other; the polarities of magnetic fields
applied to the magneto-optical element, through the coils opposing
each other with respect to the magneto-optical element, are reverse
to each other; assuming that the middle portion of the pillar
portion regarded as a boundary, the magnetic field generated
through the one-side portion of the magnetic yoke and the magnetic
field generated through the other-side portion magnetic yoke
cooperate to act on the magneto-optical element; and a pair of the
coils diagonally opposing each other with respect to the
magneto-optical element configure a set and are driven by a common
power supply unit.
With the foregoing configuration, it is preferable that, in the
case where the direction of a saturation combined magnetic field is
parallel to the optical axis of the magneto-optical element, the
Faraday rotation angle of the magneto-optical element is set to
127.3.degree. or larger, and the coils are driven by monopolar
power supply units.
As is the case with the magneto-optical device according to the
first aspect of the present invention described above, the
magneto-optical device according to the second aspect of the
present invention can be utilized as a Faraday rotator in a
variable optical attenuator or an optical switch. Further, by
generating magnetic fields by utilizing electromagnets only,
without utilizing any fixed magnetic field of a permanent magnet,
the magnetization direction can instantaneously be changed, whereby
downsizing and speedup can be implemented. Since, regardless of the
direction of the combined magnetic field, no large magnetic field
is required, the coils can be downsized, whereby the driving
voltage can also be reduced.
Since a pair of coils diagonally opposing each other with respect
to a magneto-optical element is configured and the coils in the
pair are wired in parallel or in series and driven by a common
power supply unit, the drive of the coils can efficiently be
implemented with a small number of power supply units, whereby the
peripheral circuitry can be simplified. In particular, if coils
that are connected in parallel are driven, the voltage can be
reduced. Moreover, if, in the case where the direction of a
saturation combined magnetic field is parallel to the optical axis
of the magneto-optical element, the Faraday rotation angle of the
magneto-optical element is set to 127.3.degree. or larger, the
Faraday rotation angle can be varied over the range from 0.degree.
to 90.degree. even though the coils are driven by monopolar power
supply units.
By employing the magneto-optical device having a configuration in
which two magnetic yokes made of a high-magnetic-permeability
material and having 2n (where n.gtoreq.2) pillar portions
protruding perpendicularly from one side of a tabular portion, are
combined in such a way that, the foremost surfaces of the pillar
portions of one magnetic yoke butt against the foremost surfaces of
the corresponding pillar portions of the other magnetic yoke, or a
configuration in which a magnetic yoke made of a
high-magnetic-permeability material and having tabular portions
that are spaced apart from and opposing each other and 2n pillar
portions arranged between the tabular portions is utilized, a
larger magnetic field can be applied to the magneto-optical
element.
[The Third Aspect of the Present Invention]
According to the third aspect of the present invention, there is
provided a magneto-optical device comprising: a magnetic yoke
having a tabular portion made of a semi-hard magnetic material and
at least three pillar portions, protruding from one side of the
tabular portion, that are made of a high-magnetic-permeability
material; coils wound around the pillar portions; and a
magneto-optical element arranged in an open-magnetic-circuit space
surrounded by the respective top-end portions of the pillar
portions, wherein magnetic fields generated through the coils are
applied to the magneto-optical element.
In the magneto-optical device configured as described above, when
respective currents are supplied to the coils, a magnetic field is
generated not only in the open-magnetic-circuit space but also in
the magnetic yoke, whereby both the tabular portion and the pillar
portions that configure the magnetic yoke are magnetized. When
exciting currents are cut off, the magnetization in the pillar
portions made of a soft-magnetic material tends to transit in a
direction along the hysteresis curve so as to lose its own
magnetization; in contrast, the tabular portion made of a semi-hard
magnetic material exhibit a residual magnetization that magnetizes
the pillar portions, whereby a combined magnetic field is formed in
the open-magnetic-circuit space. The combined magnetic field is
different in magnitude from that in the case where the currents are
applied, but the same in direction. Accordingly, by controlling the
ratios of the magnetomotive force generated through the coils,
thereby controlling the condition of magnetization to be maintained
in the tabular portion, the direction of the combined magnetic
field that remains in the open-magnetic-circuit space, after the
exciting currents are cut off, can be controlled so as to be in an
arbitrary direction.
The magnetic yoke may have a structure in which a tabular portion
made of a high-magnetic-permeability material and at least three
pillar portions, protruding from one side of the tabular portion,
that are made of a semi-hard magnetic material, are incorporated.
In the case of this configuration, after the exciting currents are
cut off, magnetization remains in the pillar portions, and the
residual magnetization forms a combined magnetic field in the
open-magnetic-circuit space.
By arranging a light polarizer and a light analyzer before and
after the foregoing magneto-optical device in the light path
thereof and controlling an attenuation value of output optical
power versus input optical power, a self-latching variable optical
attenuator can be configured. Moreover, by arranging the
magneto-optical device, a light polarizer, a light analyzer, and a
wavelength plate in a predetermined order and controlling optical
separation ratio through the magneto-optical device, a
self-latching variable optical splitter can be configured.
Further, according to the third aspect of the present invention,
there is provided a magneto-optical device comprising: a magnetic
yoke having a block-shaped base portion made of a semi-hard
magnetic material and at least four pillar portions, made of a
high-magnetic-permeability material, extending from the base
portion to the vicinity of a space to be an open-magnetic-circuit
space; coils wound around the pillar portions; and a
magneto-optical element arranged in the open-magnetic-circuit space
surrounded by the respective top-end portions of the pillar
portions, wherein magnetic fields generated through the coils are
applied to the magneto-optical element, in three or more
directions. In the case of this configuration, the direction of the
combined magnetic field applied to the magneto-optical element can
be controlled so as to be in an arbitrary direction in a
3-dimentional space, and the magnetic direction can be maintained
after the exciting currents are cut off.
As is the case with the magneto-optical devices according to the
first and second aspects described above, the magneto-optical
device according to the third aspect of the present invention can
be utilized as a Faraday rotator in a variable optical attenuator,
a variable optical splitter, or an optical switch. Further, fixed
magnetic field of a permanent magnet is not required, therefore, by
generating variable magnetic fields by utilizing electromagnets
only, downsizing of the magneto-optical device can be implemented.
Because, regardless of the direction of the combined magnetic
field, no large magnetic field is required, the coils can be
downsized, whereby the driving voltage can also be reduced.
Moreover, by controlling the directions and the values of the
respective currents applied to the coils, the direction (the
magnetization direction of the magneto-optical element) of the
combined magnetic field formed in the open-magnetic-circuit space
can be controlled to be in an arbitrary direction, whereby the
Faraday rotation angle can arbitrarily be adjusted. Furthermore, by
forming with a semi-hard magnetic material the tabular portion or
the pillar portion of the magnetic yoke, even after the exciting
currents are cut off, the direction of the combined magnetic field
that remains in the open-magnetic-circuit space can be controlled
so as to be in an arbitrary direction, whereby the Faraday rotation
angle can be maintained so as to be in an arbitrary condition.
In the case where the magneto-optical device is utilized as a
Faraday rotator in a variable optical attenuator, the value of
optical attenuation can be maintained to be in an arbitrary
condition even after the exciting currents are cut off. Still
moreover, in the case where the magneto-optical device is utilized
as a Faraday rotator in a variable optical splitter, the separation
ratio for input light can be maintained to be in an arbitrary
condition after the exciting currents are cut off.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is an exploded perspective view illustrating Embodiment 1
of a magneto-optical device according to the present invention;
FIG. 1B is a side view of the magneto-optical device in FIG.
1A;
FIG. 1C is a plan view of the magneto-optical device in FIG.
1A;
FIG. 2 is a view for explaining the operation of the
magneto-optical device in FIG. 1A;
FIG. 3 is a graph representing a relationship between the Faraday
rotation angle and the current, with the magnetization direction
utilized as a parameter;
FIG. 4 is a view illustrating a configuration of a variable optical
attenuator or an optical switch;
FIG. 5 is an explanatory graph representing an example of the
response performance of a magneto-optical device;
FIG. 6A is an explanatory view illustrating Embodiment 2 of a
magneto-optical device according to the present invention;
FIG. 6B is an explanatory view illustrating a condition in which
air-core coils are mounted on the magnetic yoke in FIG. 6A;
FIG. 6C is an explanatory view illustrating a condition in which a
magneto-optical element is arranged in the open-magnetic-circuit
space in FIG. 6B;
FIG. 7A is a diagram for explaining arrangement of coils;
FIG. 7B is a chart for explaining magnetic fields to be
applied;
FIG. 8 is a set of diagrams illustrating the relationship between a
group of magnetic poles and the direction of a magnetic field to be
applied;
FIG. 9A is a circuit diagram illustrating wiring of coils and power
supply units, in the case where two coils each are connected in
parallel;
FIG. 9B is a circuit diagram illustrating wiring of coils and power
supply units, in the case where two coils each are connected in
series;
FIG. 10A is a graph representing a relationship between the Faraday
rotation angle and the current, in the case where a magneto-optical
element having .theta..sub.f0 of 90.degree. is utilized;
FIG. 10B is a graph representing a relationship between the Faraday
rotation angle and the current, in the case where a magneto-optical
element having .theta..sub.f0 of 127.3.degree. is utilized;
FIG. 11A is an explanatory view illustrating an example of another
structure of a magnetic yoke;
FIG. 11B is an explanatory view illustrating a condition in which
air-core coils are mounted on the magnetic yoke in FIG. 11A;
FIG. 11C is an explanatory view illustrating a condition in which a
magneto-optical element is arranged in the open-magnetic-circuit
space in FIG. 11B;
FIG. 12 is a chart representing another example of arrangement of
coils and magnetic fields to be applied;
FIG. 13 is a graph representing another example of the relationship
between the Faraday rotation angle and the current;
FIG. 14 is an explanatory view illustrating an example of another
structure of a magneto-optical device;
FIG. 15A is an explanatory view illustrating an example of further
another structure of a magneto-optical device;
FIG. 15B is an explanatory view illustrating a condition in which
the magnetic yokes in FIG. 15A are bonded;
FIG. 15C is an explanatory view illustrating a condition in which a
magneto-optical element is arranged in the open-magnetic-circuit
space in FIG. 15B;
FIG. 16A is an explanatory diagram illustrating arrangement of
coils;
FIG. 16B is a circuit diagram illustrating wiring of coils and
power supply units, in the case where two coils each are connected
in parallel;
FIG. 16C is a circuit diagram illustrating wiring of coils and
power supply units, in the case where two coils each are connected
in series;
FIG. 17A is an explanatory view illustrating an example of another
structure of a magneto-optical device;
FIG. 17B is a perspective view illustrating a condition after the
magneto-optical device in FIG. 17A has been assembled;
FIG. 18 is an explanatory diagram illustrating an application
example of a magneto-optical device applied to a variable optical
attenuator;
FIG. 19A is an explanatory graph representing an example of the
optical attenuation properties of a variable optical
attenuator;
FIG. 19B is an explanatory graph representing an example of the
response properties of a variable optical attenuator;
FIG. 20 is an explanatory view illustrating Embodiment 3 of a
magneto-optical device according to the present invention;
FIG. 21A is an explanatory view illustrating an example of the
magnetization direction of a magnetic yoke and the direction of a
combined magnetic field, in the case where currents are
applied;
FIG. 21B is an explanatory view illustrating an example of the
magnetization direction of a magnetic yoke and the direction of a
combined magnetic field, in the case where currents are cut
off;
FIG. 22 is an explanatory view illustrating an example of another
structure of a magnetic yoke;
FIG. 23 is an explanatory view illustrating an example of further
another structure of a magnetic yoke;
FIG. 24 is an explanatory chart representing an example of the
angle between the direction of a combined magnetic field and the
traveling direction of a ray;
FIG. 25A is a graph representing an example of the relationship
between the direction of a combined magnetic field and the
current;
FIG. 25B is a graph representing an example of measurement on the
Faraday rotation angle, in the case where currents are applied and
after the currents are cut off;
FIG. 26 is an explanatory diagram illustrating an example of a
magneto-optical device applied to a variable optical
attenuator;
FIG. 27 is an explanatory graph representing an example of the
optical attenuation properties of the variable optical attenuator
in FIG. 26; and
FIG. 28 is an explanatory diagram illustrating an example of a
magneto-optical device applied to a variable optical splitter.
TABLE-US-00001 Description of Symbols 10 MAGNETIC YOKE 12 COIL 14
MAGNETO-OPTICAL ELEMENT 16 TABULAR PORTION 18 PILLAR PORTION 110
TABULAR PORTION 112 PILLAR PORTION 114 MAGNETIC YOKE 116 COIL 120
MAGNETO-OPTICAL CRYSTAL 122, 123, 126, and 127 POWER SUPPLY UNIT
124 and 128 CONTROL DEVICE 210 MAGNETIC YOKE 211 TABULAR PORTION
212 PILLAR PORTION 215 COIL 220 MAGNETO-OPTICAL ELEMENT
MODE FOR CARRYING OUT THE INVENTION
EMBODIMENT 1
Embodiment 1 of a magneto-optical device according to the present
invention will be explained with reference to FIGS. 1A to 5. In the
case of the most simple configuration of a magneto-optical device
in Embodiment 1, a single-piece magnetic yoke made of a
high-magnetic-permeability material is utilized in which
quadrangular pillar portions are protruding perpendicularly and
identically oriented, from the vicinities of the four corners of a
square tabular portion, respective coils are wound around the
quadrangular pillar portions, and a magneto-optical element is
arranged in an open-magnetic-circuit space surrounded by the
top-end portions of the four quadrangular pillar portions. Besides,
variable magnetic fields through the coils are applied to the
magneto-optical element.
FIGS. 1A to 1C illustrate an example of a magneto-optical device
according to the present invention. FIG. 1A is an exploded
perspective view; FIGS. 1B and 1C are a side view and a plan view,
respectively. The magneto-optical device includes a magnetic yoke
10, coils 12 wound around the magnetic yoke 10, and a
magneto-optical element 14. The magnetic yoke 10 has a structure in
which a square tabular portion 16 and four square pillar portions
18 that are equal in length and protruding perpendicularly and
identically oriented, at the four corners of the square tabular
portion 16, are continuously integrated, and made of a
high-magnetic-permeability material. The coils 12 are wound around
the respective pillar portions 18. The magneto-optical element 14
is arranged in an open-magnetic-circuit space surrounded by the
top-end portions of the four pillar portions 18. Accordingly, a
magnetic field generated by the coils 12 is applied to the
magneto-optical element 14.
As the high-magnetic-permeability material, for example, Ni--Zn
system ferrite is utilized. The magnetic yoke 10 has a structure
formed by integrally molding the high-magnetic-permeability
material in a predetermined form and sintering the molded material.
The performance of the magnetic yoke 10 is enhanced by utilizing
ferrite single crystal. In this case, the magnetic yoke 10 is
produced by cutting out a portion of ferrite in a predetermined
shape from a block ferrite. As the magneto-optical element, for
example, a bismuth-substituted rare-earth iron-garnet single
crystal is utilized. The single crystal can be grown through the
LPE (liquid-phase epitaxy) method. Additionally, a YIG (yttrium
iron garnet) single crystal may be utilized.
In the case of the magneto-optical device, by controlling the
directions and/or values of electric currents supplied to four
coils, the magnetization direction of the magneto-optical element
can be changed.
FIG. 2 is a set of plan views for explaining the operation of a
magneto-optical device according to the present invention. The
respective top ends of the pillar portions of the magnetic yoke are
indicated by reference characters a, b, c, and d; the respective
coils that correspond to the top ends and are wound around the
pillar portions are indicated by similar reference characters
(coils 12a to 12d). In this situation, it assumed that, as
indicated by an arrow, light passes through the magneto-optical
element, from the left-hand side of FIG. 2 to the right-hand side.
The coils 12a and 12b are wound in series. Besides, a and b have
opposite magnetic polarities. The coils 12c and 12d are also wound
in series. Further, c and d have opposite magnetic polarities.
Magnetization directions are indicated by white arrows.
(In-Plane Magnetization)
Single-direction currents flow through the coils 12a and 12b and
through the coils 12c and 12d. Accordingly, a and b are magnetized
to N and S poles, respectively; similarly, c and d are magnetized
to N and S poles, respectively. In consequence, a magnetic field
perpendicular to the optical axis is applied to the magneto-optical
element 14, i.e., in-plane magnetization (magnetization whose
direction is parallel to the incident/exit plane of the
magneto-optical element) is generated.
(45-Degree Magnetization)
A single-direction current flows through the coils 12a and 12b
only. No current is applied to 12c and 12d. Accordingly, a and b
are magnetized to N and S poles, respectively; inconsequence, a
magnetic field whose direction has a gradient of 45.degree. with
respect to the optical axis is applied to the magneto-optical
element 14, i.e., 45-degree magnetization (magnetization whose
direction has a gradient of 45.degree. with respect to the
incident/exit plane of the magneto-optical element) is
generated.
(Perpendicular Magnetization)
A single-direction current flows through the coils 12a and 12b, and
a single-direction current whose direction is reverse to that in
the case of the in-plane magnetization flows through the coils 12c
and 12d. Accordingly, a and b are magnetized to N and S poles,
respectively; c and d are magnetized to S and N poles,
respectively. In consequence, a magnetic field whose direction is
parallel to the optical axis is applied to the magneto-optical
element 14, i.e., perpendicular magnetization (magnetization whose
direction is perpendicular to the incident/exit plane of the
magneto-optical element) is generated.
At any rate, an electric current whose direction is always the same
flows through the coils 12a and 12b, whereby the top ends a and b
are magnetized always in the same polarity (e.g., a is magnetized
always to N, and b, to S pole). From the state of in-plane
magnetization to the state of the 45-degree magnetization, a
single-direction current flows also through the coils 12c and 12d;
by reducing the value of the current, arbitrary magnetization
directions can be realized. From the state of 45-degree
magnetization to the state of the perpendicular magnetization, a
reverse-direction current flows through the coils 12c and 12d; by
increasing the value of the current, arbitrary magnetization
directions can be realized. As described above, by controlling the
values and directions of the currents applied to the coils, the
direction of the magnetic field can be controlled.
FIG. 3 is a graph representing an example of measurement results
with regard to the relationship between the Faraday rotation angle
and the coil-current value, with the magnetization direction
utilized as a parameter. Although depending on the structure and
the material of the magnetic yoke, it can be seen that, when the
value of the coil current is larger than a specific value, the
Faraday rotation angle is saturated to a constant value.
FIG. 4 is an explanatory view illustrating an example of a variable
optical attenuator according to the present invention. A
configuration is employed in which, before and after the
magneto-optical device 20, a first light polarizer 22 and a second
light polarizer 24 are arranged along the light path. The
magneto-optical device 20 may be identical to that illustrated in
FIGS. 1A to 1C; for simplicity, corresponding members are
designated by the same reference characters.
In this situation, the magneto-optical element 14 is made thick
enough to cause a Faraday rotation angle of 45.degree. or larger.
In addition, the first light polarizer 22 and the second light
polarizer 24 are linear polarizers, such as an absorption-type
polarizer, polarization glass (brand names "Polarcor", "CUPO", and
the like), or a multilayer-type polarizer, and arranged in such a
way that their optical axes are within planes that are parallel to
each other and have a predetermined angle difference (e.g., a
predetermined angle difference of 45.degree. or larger between the
optical axes, when viewed with respect to the light path) between
them.
By controlling through the magneto-optical device 20 the Faraday
rotation angle of the magneto-optical element 14 over the range
from -45.degree. to +45.degree., the amount of light output from
the second light polarizer 24 versus the amount of input light to
the first light polarizer 22 is variably controlled.
Because, without utilizing any fixed magnetic field generated by a
permanent magnet, the magnetic field for a variable optical
attenuator configured as described above is made up merely of
variable magnetic fields generated by electromagnets, the
magnetization direction can instantaneously be changed, whereby the
operation of the variable optical attenuator is speeded up. In
addition, because the magnetization direction can arbitrarily be
controlled, the range from -45.degree. to +45.degree. can be
utilized so that the 90-degree variable amount in the Faraday
rotation angle, which is required of an optical attenuator, is
obtained; therefore, in comparison to a conventional configuration,
the magneto-optical element is required to have merely half of the
rotation angle (i.e., a thickness corresponding to 45-degree
rotation angle).
The conditions obtained are listed below under which, by utilizing
a magnetic yoke that is made of Ni--Zn ferrite having a magnetic
permeability of 2000 and 2.8 mm by 2.8 mm by 4.5 mm (length by
width by height) in size, a magnetic-field strength of 12000 A/m
(150 Oe), which is required to magnetically saturate the
magneto-optical element or reverse the magnetization direction, is
obtained: Number of windings: 130 Coil resistance (diameter of
conductor .phi.=50 .mu.m): 5.17.OMEGA. Magnetomotive force: 11.9 AT
Current value: 96 mA Power consumption: 48 mW Time constant: 50.1
.mu.sec
FIG. 5 is a graph representing an example of measurement result on
the response speed of the magnetic circuit. The duration between
the instant when the input voltages to be applied to both sets of
coils were concurrently switched and the instant when the light
output became stable was measured. It can be seen in FIG. 5 that
the response speed was approximately 40 .mu.sec. For reference's
sake, with a conventional optical attenuator, the response speed
was approximately 200 .mu.sec. The optical attenuator according to
the present invention does not require a permanent magnet;
therefore, the response speed is raised.
As is the case with FIG. 4, an optical switch is configured in such
a way that, before and after a magneto-optical device, respective
light polarizers are arranged along the light path of the
magneto-optical device. In the case of an optical switch, it is
desirable to utilize a magneto-optical element having a
self-latching function. That is because, thanks to the
self-latching function, the optical switch can maintain the state
even though the coil current is cut off. As a magneto-optical
element having a self-latching function, for example, a
bismuth-substituted rare-earth iron garnet single crystal having
residual magnetization can be utilized; by, without applying heat
treatment, directly utilizing a film grown through the LPE method,
the self-latching function is demonstrated. By switching Faraday
rotation angles through the magneto-optical device, output light
versus input light can be switchably controlled. Accordingly, it is
not necessary that, as a conventional magneto-optical device, the
magnetic-yoke material is limited to a semi-hard magnetic material.
Therefore, the magnetic field generated through the coils and the
magnetic yoke may be a critical mass for reversing the
magnetization of the magneto-optical element; as a result, it is
possible to downsize the device and to reduce the power
dissipation.
Moreover, the magneto-optical device according to the present
invention has a structure in which no permanent magnet is utilized,
the leakage magnetic field is small, and the magnetic fields
converge on the magneto-optical element only; therefore, even
though a plurality of magneto-optical devices are provided in
parallel, no magnetic interference occurs, whereby an array
structure is readily enabled. Variable optical attenuators, optical
switch arrays, and the like can be realized, by utilizing the
magneto-optical device.
In addition, although, in Embodiment 1, a configuration is employed
in which four pillar portions are provided on the tabular portion,
the present invention is not limited to the configuration; e.g., a
configuration may also be employed in which three, or five or more
pillar portions are provided. If at least three pillar portions are
provided on the tabular portion, by controlling respective
directions and values of the currents supplied to the coils wound
around the pillar portions, the combined magnetic field can
arbitrarily be oriented, as is the case with four pillar portions.
Moreover, the shape of the tabular portion is not limited to a
square; e.g., other polygons, a circle, or the like may be
employed.
EMBODIMENT 2
Embodiment 2 of a magneto-optical device according to the present
invention will be explained with reference to FIGS. 6A to 19B. In
the case of the most simple configuration of a magneto-optical
device in Embodiment 2, as illustrated in FIGS. 6A to 6C, a
single-piece magnetic yoke 114 made of a high-magnetic-permeability
material is utilized in which quadrangular pillar portions 112 are
protruding perpendicularly and in the same direction, from the
vicinities of the four corners of a square tabular portion 110
(refer to FIG. 6A). Respective coils 116 are wound around the
quadrangular pillar portions 112 (refer to FIG. 6B); a
magneto-optical element 120 mounted on a non-magnetic holding
member 118 is arranged in an open-magnetic-circuit space surrounded
by the top ends of the four quadrangular pillar portions 112 (refer
to FIG. 6C). Besides, the magneto-optical device is configured in
such a way that variable magnetic fields generated through the
coils 116 are applied to the magneto-optical element 120.
As illustrated in FIG. 7A, the respective coils are designated by
Reference characters a to d. In accordance with magnetic poles
that, through electric currents being applied to the coils, appear
at the respective top ends of the pillar portions, the direction of
a combined magnetic field to be applied to the magneto-optical
element 120 can be changed. As illustrated in FIG. 7B, in the case
where, through the coils a and b, magnetic poles S and N are
respectively generated, a magnetic field H.sub.1 occurs oriented at
-45.degree. from the optical axis that passes through the
magneto-optical element 120; in contrast, in the case where,
through the coils c and d, magnetic poles N and S are respectively
generated, a magnetic field H.sub.2 occurs oriented at +45.degree.
from the optical axis that passes through the magneto-optical
element 120. In the case where both H.sub.1 and H.sub.2 are
applied, a combined magnetic field HC generated through H.sub.1 and
H.sub.2 is applied to the magneto-optical element 120. Accordingly,
by controlling the values or the directions of the electric
currents applied to the respective coils, the combined magnetic
field HC can be applied in any direction.
By controlling the values or the directions of the electric
currents applied to the respective coils, it is possible to make
the direction of the combined magnetic field turn 360 degrees. FIG.
8 illustrates the typical directions of the combined magnetic
field. In FIG. 8, conditions in the case where the combined
magnetic field is changed by 45.degree. successively are
represented in states 1 to 8. It can be seen from FIG. 8 that, when
magnetic fields are generated by applying electric currents to the
coils, two poles, at the top ends of the pillar portions, that are
generated through the coils (a and b, or c and d) opposing each
other with respect to the magneto-optical element 120 have
respective polarities that are reverse to each other. Paying
attention to the fact, it can be seen that the two coils can be
connected to a monopolar power supply unit in such a way as to
generate respective polarities that are reverse to each other.
Accordingly, even through there are four coils, two coils can be
driven as a pair by a common power supply unit; therefore, only two
power supply units are required, and a relatively simple control
system may be utilized.
FIGS. 9A and 9B illustrate examples of coil-connection methods.
FIG. 9A illustrate an example in which the coils a and b are
connected in parallel and connected to a first common power supply
unit 122, the coils c and d are connected in parallel and connected
to a second common power supply unit 123, and control is
implemented by a control device 124. FIG. 9B illustrate an example
in which the coils a and b are connected in series and connected to
a first common power supply unit 126, the coils c and d are
connected in series and connected to a second common power supply
unit 127, and control is implemented by a control device 128. In
this situation, reference characters a to d correspond to those in
FIG. 7A. Each power supply unit may be a variable voltage source or
a variable current source. By controlling the coil currents by
means of the control devices 124 and 128 such as a CPU, the
magnitude and the direction of the combined magnetic field can be
controlled. The combined magnetic field may have constant magnitude
to such an extent as can magnetically saturate the magneto-optical
element, regardless of the magnetization direction. In the present
invention, because there is no fixed magnetic field generated by a
permanent magnet, it is not necessary to excessively magnify the
combined magnetic field, whereby the number of coil windings can be
reduced, and the magneto-optical device can be downsized.
When a plurality of coils that are in parallel with one another is
connected to a power supply unit, the total resistance value is
reduced. For example, if two coils having resistance R are
connected in parallel, the total resistance R.sub.p is R/2; if two
coils having resistance R are connected in series, the total
resistance R.sub.s is 2R; therefore, if the total resistances are
compared with each other, R.sub.p is a quarter of R.sub.s. In the
case of parallel connection, the current I that flows in each coil
is V/R. In the case of series connection, the current I that flows
in each coil is V/2R; should the same current I is required in both
the series connection and the parallel connection, the voltage to
be applied in the case of the parallel connection is half of that
in the case of the series connection, whereby low-voltage drive is
enabled. Because coils that configure the pair are the same in the
number of windings and approximately the same in the length of the
wire, their resistances are approximately the same; therefore, in
the case of the parallel connection, approximately the same current
can flows through each coil.
Magneto-optical devices illustrated in FIGS. 6A to 6C can be
produced, for example, in the following way. By cutting notches,
from two directions that are perpendicular to each other, in a
rectangular-parallelepiped block made of a
high-magnetic-permeability material such as NiCuZn-system, a
magnetic yoke 114 having four integrated pillar portions 112 can be
created (refer to FIG. 6A). It goes without saying that the
magnetic yoke 114 may be formed through press molding and baked to
a desirable shape. Respective air-core coils 116 that have
preliminarily been produced are mounted and fixed, through an
adhesive or the like, on the four pillar portions 112 (refer to
FIG. 6B). In this case, if the air-core coil 116 is made of an
enamel-coated wire material, the air-core coil 116 may be fixed on
the pillar portion 112 by coating an organic solvent such as ethyl
alcohol on the air-core coil 116 to dissolve the enamel after
mounting the air-core coil 116 on the pillar portion 112. Because,
in the foregoing configuration, the magnetic yoke 114 has a unified
structure, magnetic resistance thereof can be suppressed to a
minimum, whereby a high-efficiency magnetic circuit can be
obtained. Next, the stage 118, made of a non-magnetic material, to
which the magneto-optical element 120 is adhesively fixed, is fixed
through an adhesive or the like on the top ends of the pillar
portions 112 (refer to FIG. 6C). As the magneto-optical element
120, for example, a bismuth-substituted rare-earth iron-garnet
single crystal is utilized. The single crystal can be grown through
the LPE (liquid-phase epitaxy) method. Additionally, a YIG (yttrium
iron garnet) single crystal may be utilized.
Besides, two pairs of the coils that are arranged diagonally
opposing each other, with respect to the magneto-optical element,
are connected in parallel (refer to FIG. 9A) or in series (refer to
FIG. 9B) to the power supply unit. In this situation, the
polarities of magnetic fields to be applied to the magneto-optical
element are made reverse to each other. Compared with a method in
which, with four coils connected to respective power supply units,
the four power supply units are controlled to adjust the magnitude
and direction of the combined magnetic field, the control is far
facilitated, and costs are reduced.
As illustrated in FIG. 7B, the magnetic field generated through a
first circuit is designated by H.sub.1; the magnetic field
generated through a second circuit is designated by H.sub.2.
H.sub.1 is oriented at -45.degree. from the incident optical axis,
and H.sub.2, +45.degree.. The foregoing magnetic-field directions
are angles obtained through magnetic yokes, as illustrated in FIGS.
6A to 6C, whose structures are inexpensive and readily producible.
In this case, the direction .theta..sub.c of the combined magnetic
field is given by the following equation:
.theta..sub.c=45-tan.sup.-1(H.sub.1/H.sub.2) Because the magnitudes
of the magnetic fields generated through the respective circuits
are proportional to the values of the currents, H.sub.1 and H.sub.2
in the above equation can be replaced by a current I.sub.1 that
flows in the first circuit and a current I.sub.2 that flows in the
second circuit, respectively. Because the following equation is
given, it can be seen that, by controlling the currents I.sub.1 and
I.sub.2, a desired Faraday rotation angle can be obtained.
.theta..sub.f=.theta..sub.f0.times.cos .theta..sub.c
In the case where a magneto-optical is utilized within the range,
of the Faraday rotation angle .theta..sub.f, from 0.degree. to
.theta..sub.fMAX, mostly utilized is a magneto-optical element
whose Faraday rotation angle .theta..sub.f0 in the case where the
direction of the saturation combined magnetic field is in parallel
with the optical axis is equal to .theta..sub.fMAX. FIG. 10A is a
graph representing a relationship between .theta..sub.f and the
current I.sub.1 or I.sub.2, in the case where a magneto-optical
element having .theta..sub.f0 of 90.degree. is utilized. In this
case, when the direction .theta..sub.c of the combined magnetic
field is requested to be smaller than 45.degree., it is necessary
to reverse the polarity of H.sub.1. In other words, it is necessary
to make the current flow reversely. It can be seen from FIG. 10A
that the polarity of I.sub.1 reverses at the .theta..sub.f of
approximately 64.degree.. The fact is because the maximal Faraday
rotation angle .theta..sub.fMAX in the case of the actual use is
equal to .theta..sub.f0. The above configuration enables
.theta..sub.f to be varied from 0.degree. to 90.degree.; however,
it requires bipolar power supply units, thereby raising the costs.
Additionally, there is an inflection point in the current I.sub.2,
whereby control of the currents is rendered slightly
complicated.
The problem can be solved, by making the Faraday rotation angle
.theta..sub.f0 of the magneto-optical element in the case where the
direction of the saturation combined magnetic field is in parallel
with the optical axis be larger than .theta..sub.fMAX so that
.theta..sub.f becomes equal to .theta..sub.fMAX or larger when the
magnetic field to be applied consists of H.sub.2 only. For example,
in the case where, as the foregoing example, H.sub.2is oriented at
45.degree. from the incident optical axis, the magnetic field to be
applied consists of H.sub.2 only, and .theta..sub.f0 is 90.degree.,
.theta..sub.f is given the following equation:
.theta..sub.f=90.times.cos 45=63.6.degree. Accordingly,
.theta..sub.f is small by 26.4.degree. in comparison to the
required .theta..sub.fMAX, 90.degree.. In this case, if a
magneto-optical element having a large .theta..sub.f0, e.g.,
127.3.degree., is utilized, .theta..sub.f is given by the following
equation; therefore, without applying a magnetic field reverse to
H.sub.1, .theta..sub.fMAX can be satisfied.
.theta..sub.f=127.3.times.cos 45=90.degree. In other words, if the
following equation is yielded, the foregoing problem can be solved.
.theta..sub.fMAX=.theta..sub.f0.times.cos .theta..sub.c
FIG. 10B is a graph representing a relationship between the Faraday
rotation angle .theta..sub.f and the current I.sub.1 or I.sub.2, in
the case where a magneto-optical element is utilized that has the
Faraday rotation angle .theta..sub.f0 of 127.3.degree. when the
direction of the saturation combined magnetic field is in parallel
with the optical axis. From FIG. 10B, it can be seen that there is
no reversal of polarity in the current I.sub.1 and no inflection
point in the current I.sub.2. Accordingly, the foregoing
configuration makes it possible to utilize monopolar power supply
units and facilitates the control of the currents.
FIGS. 11A to 11C are views illustrating another example of the
magnetic yoke. The magnetic yoke is configured in such a way that
through-holes 132 are provided at the four corners of a square
tabular portion 130 made of ferrite or the like, cylindrical
members 134 made of ferrite or the like are inserted into the
through-holes 132 (refer to FIG. 11A) and fixed thereto through an
adhesive or the like (refer to FIG. 11B) As described above, it is
possible to produce relatively readily a magnetic yoke that is
substantially unified. A magneto-optical element 138 is fixed in
such a way as to be situated in the center of the top ends of the
four cylindrical members 134. For that purpose, for example,
respective holes (unillustrated) are provided at the four corners
of a square-tabular non-magnetic stage 140 so that the cylindrical
members are automatically positioned at and fixed to the respective
holes, and with the holes as reference points, a hole is provided
at which the magneto-optical element 138 is positioned and fixed;
in this way, the magneto-optical element 138 can readily be fixed
in the center of the top ends of the four cylindrical members 134
(refer to FIG. 11C).
As illustrated in FIG. 12, in order to further simplify the control
of the currents, the first magnetic field H.sub.1 is made to be in
a direction, perpendicular to the incident optical axis, in which
the Faraday rotation is 0.degree., and the second magnetic field
H.sub.2 is set to be tilted by .theta..sub.c from the incident
optical axis. Accordingly, if .theta..sub.f is requested to be
zero, only I.sub.1 is supplied; and if .theta..sub.f is requested
to be .theta..sub.fMAX, only I.sub.2 is supplied. FIG. 13 is a
graph representing a relationship between the Faraday rotation
angle .theta..sub.f and the current I.sub.1 or I.sub.2, in the case
where a magneto-optical element is utilized that has the Faraday
rotation angle .theta..sub.f0 of 127.3.degree. when the direction
of the saturation combined magnetic field is in parallel with the
optical axis. The angle .theta..sub.c is 45.degree.. It can be seen
that both I.sub.1 and I.sub.2 can linearly be varied for
.theta..sub.f. Because, in this case,
.theta..sub.h=90-.theta..sub.c, .theta..sub.f=.theta..sub.fMAX, and
.theta..sub.c=cos.sup.-1(.theta..sub.fMAX/.theta..sub.f0),
.theta..sub.h can be rendered by the following equation:
.theta..sub.h=90-cos.sup.-1(.theta..sub.fMAX/.theta..sub.f0)=sin.sup.-1(.-
theta..sub.fMAX/.theta..sub.f0) In other words, if the above
equation is satisfied, the control can be simplified.
As illustrated in FIG. 14, a magnetic yoke that can embody the
above equation can be produced by cutting off three pertinent parts
(two parallel parts and one part perpendicular to the others) of a
rectangular-parallelepiped block made of a
high-magnetic-permeability material. In a magnetic yoke 144,
compared with the magnetic-pole positions for H.sub.1, the
magnetic-pole positions for H.sub.2 are far from the center of a
magneto-optical element 146, whereby the magnitude of the magnetic
field H.sub.2 to be applied to the magneto-optical element 146 is
reduced; however, the reduced magnitude can be addressed, e.g., by
increasing the number of windings. In addition, instead of a
single-piece structure, by providing through-holes at predetermined
positions in the tabular portion and inserting and fixing pillar
members into the through-holes, the magnetic yoke may be produced,
as is the case with FIGS. 11A to 11C.
FIGS. 15A to 15C are explanatory views further illustrating another
example of a magneto-optical device according to the present
invention. Two magnetic yokes 150 as illustrated in FIGS. 6A to 6C
are arranged opposing each other. In other words, each of the two
magnetic yokes 150 has a single-piece structure, made of a
high-magnetic-permeability material such as ferrite, that
incorporates an approximately square tabular portion 152 and four
pillar portions 154 protruding perpendicularly from four corners in
the one surface of the tabular portion 152. Besides, coils 156 are
wound around the respective pillar portions 154 (refer to FIG.
15A). The two magnetic yokes 150 around which the coils 156 are
wound as described above are integrated in such a way that the
foremost surfaces of the pillar portions 154 of the one magnetic
yoke 150 butt against the foremost surfaces of the pillar portions
154 of the other magnetic yoke 150, and fixed through an adhesive
or the like (refer to FIG. 15B). A cylindrical support 162 to which
a magneto-optical element 160 is adhered is inserted into a
through-hole provided in the center of a non-magnetic stage 158,
and fixed therein (refer to FIG. 15B); the stage 158 is fixed to
the magnetic yoke 150, through adhesion or the like, in such a way
that the magneto-optical element 160 is situated in the
open-magnetic-circuit space surrounded by the top ends of the
respective pillar portions 154 (refer to FIG. 15C).
Accordingly, the magneto-optical device is configured in such a way
that magnetic fields generated by the coils 156 are applied to the
magneto-optical element 160. For that purpose, currents are applied
to the coils in such a way that the directions of magnetic fields
generated through the coils wound around the opposing pillar
portions 154 are reverse to each other. That is to say, the
currents are applied to the coils in such a way that, at the top
ends of the opposing pillar portions 154, the respective magnetic
poles generated through the respective coils 156 have the same
polarity. As a result, magnetic fluxes leak out of the top ends of
the pillar portions 154, whereby the magnetic fields efficiently
act on the magneto-optical element 160. As is the case with FIGS.
6A to 6C, the polarities of magnetic fields applied to the
magneto-optical element 160, through the coils 156 that are
diagonally opposing each other with respect to the magneto-optical
element 160, are reverse to each other. Moreover, the
magneto-optical device is configured in such a way that the
magnetic fields generated through the upper magnetic yoke and the
lower magnetic yoke cooperatively (added to each other) act on the
magneto-optical element 160.
In addition, a set of four coils two each of which are diagonally
opposing each other with respect to the magneto-optical element is
driven by a common power supply unit. When FIG. 16A represents a
positional relationship among the coils, as illustrated in FIG.
16B, totally four coils (a.sub.1, a.sub.2, b.sub.1, and b.sub.2),
consisting of two coils vertically opposing each other with respect
the incident optical axis and two other coils each diagonally
opposing the two coils with respect to the magneto-optical element,
that are wired in parallel with one another are connected with the
one power supply unit; the residual four coils (c.sub.1, c.sub.2,
d.sub.1, and d.sub.2) are also wired in parallel with one another
and connected with the other power supply unit. Thus, the coils
vertically opposing each other with respect the incident optical
axis, e.g., a.sub.1 and a.sub.2 are set for N poles, and the coils
b.sub.1 and b.sub.2 each diagonally opposing coils a.sub.1 and
a.sub.2 with respect to the magneto-optical element are set for S
poles. Similarly, if c.sub.1 and c.sub.2 are set for S poles,
d.sub.1 and d.sub.2 are set for N poles.
Additionally, as illustrated in FIG. 16C, the four coils (a.sub.1,
a.sub.2, b.sub.1, and b.sub.2) and the residual four coils
(c.sub.1, c.sub.2, d.sub.1, and d.sub.2) that are wired in series
may be connected with the one power supply unit and the other power
supply unit, respectively. Alternatively, a method is also possible
in which two each of the coils are wired in parallel-series, or in
series-parallel, and connected with a common power supply unit. At
any rate, compared with a method in which eight coils that are
connected with respective power supply units are controlled
separately, the circuit configuration can significantly be
simplified, the costs are reduced, and the control is not rendered
complicated. A control method to obtain a desired Faraday rotation
angle is the same as that in the case where the magnetic yoke is
provided only at one side.
FIGS. 17A and 17B are explanatory views illustrating another
example of a magneto-optical device according to the present
invention. Four pillar portions 166 are arranged between tabular
portions 164 that are spaced apart from and opposing each other,
and 2 coils 156 are wound around each of the pillar portions 166.
The magneto-optical device is configured in such away that, by
arranging the magneto-optical element 160 in a space surrounded by
middle portions of the respective pillar portions 166, magnetic
fields generated by the coils 156 are applied to the
magneto-optical element 160. By providing holes 165 at the four
corners of both tabular portions 164, inserting thereinto both ends
of the respective pillar portions 166, and fixing therein the ends
of the pillar portions 166, through an adhesive or the like, a
single-piece magnetic yoke made of a high-magnetic-permeability
material can be obtained. Two coils wound around the same pillar
portion 166 generate magnetic fields that are reverse to each
other; the directions of magnetic fields applied, through the coils
diagonally opposing each other with respect to the magneto-optical
element 160, to the magneto-optical element 160 are reverse to each
other; assuming that the middle portion of the pillar portion is a
boundary, the magnetic field generated through the top-half
magnetic yoke and the magnetic field generated through the
bottom-half magnetic yoke cooperatively act on the magneto-optical
element; and the coils opposing each other with respect to the
magneto-optical element 160 is driven, as a set, by a common power
supply unit. The operation of the magneto-optical device is the
same as that of the foregoing embodiments.
In addition, the foregoing embodiments are examples in which four
pillar portions are provided in a tabular portion; however, a
structure may also be possible in which more pillar portions, e.g.,
six pillar portions are provided.
FIG. 18 is a diagram illustrating an application example of a
reflection-type variable optical attenuator including a
magneto-optical device according to the present invention. A
configuration is employed in which, before the magneto-optical
device 170, a light polarizer 172 made of a birefringence crystal
and a lens 174 are arranged along the light path, and after the
magneto-optical device 170, a mirror 176 is arranged along the
light path. The magneto-optical device 170 may be identical to that
illustrated in FIGS. 6A to 6C; for simplicity, corresponding
members are designated by the same reference characters. Light is
made to enter through an input optical fiber, and output light is
extracted through an output optical fiber. The example employs a
configuration in which coils are connected in series (refer to FIG.
9B).
FIGS. 19A and 19B are graphs representing an example of results of
measurement on a prototype. FIG. 19A represents the
optical-attenuation properties versus the current of Coil 1 (the
coils a and b) in the case where the current of Coil 2 (the coils c
and d) is fixed to 80 mA. Additionally, FIG. 19B represents the
response properties versus the step input to Coil 1 with the
attenuation value of 16 dB or 26 dB. In the case where the
attenuation value was 26 dB, the response time constant
(attenuation value: reaching down to 63%) was 24 .mu.sec.
EMBODIMENT 3
Embodiment 3 of a magneto-optical device according to the present
invention will be explained with reference to FIGS. 20 to 28. As
illustrated in FIG. 20, a magneto-optical device according to
Embodiment 3 includes a magnetic yoke 210, coils 215 wound around
the magnetic yoke 210, and a magneto-optical element 220. The
magnetic yoke 210 has a configuration in which a disk-shaped
tabular portion 211 and four inverted L-shaped pillar portions 212
protruding from a peripheral portion of the one surface of the
tabular portion 211 are incorporated, the respective pillar
portions 212 are arranged, along the circumference of the tabular
portion 211, spaced approximately the same distance apart from one
another, and the respective top ends of the pillar portions 212 are
facing the center of the pillar portions 212. The tabular portion
211 is made of, e.g., a semi-hard magnetic material such as
SUS420J2; the pillar portions 212 are made of a soft-magnetic
material such as Mn--Zn-system ferrite. The tabular portion 211 and
each of the pillar portions 212 are fixed to each other, through an
adhesive or the like.
The coils 215 are wound around the respective pillar portions 212,
and the magneto-optical element 220 is arranged in an
open-magnetic-circuit space surrounded by the top ends of the four
pillar portions 212; accordingly, the combined magnetic field
generated through the coils 215 is applied to the magneto-optical
element 220. As is the case with Embodiments 1 and 2 described
above, the polarities of magnetic fields applied to the
magneto-optical element 220, through the coils 215 that are
diagonally opposing each other with respect to the magneto-optical
element 220, are reverse to each other; the coils 215 are connected
in parallel or in series with a power supply unit. In addition, the
number of windings of the coil 215 can arbitrarily be set, and, by
changing the number of windings, the magnetomotive force of the
coil 215 can be adjusted; however, for simplicity, explanation will
be implemented on the assumption that the magnetomotive force is
adjusted through the current supplied to the coil 215.
In the magneto-optical device configured as described above, by
supplying currents to the four coils 215, the magnetomotive force
is generated in each of the coils 215. Besides, by controlling the
directions and the ratios of currents supplied to the four coils
215, a combined magnetic field (spatial magnetic field) having an
arbitrary direction can be formed in an open-magnetic-circuit space
as an objective space. When respective currents are supplied to the
coils 215, for example as illustrated in FIG. 21A, the respective
pillar portions 212 of the magnetic yoke 210 are magnetized in a
predetermined direction (e.g., in the direction indicated in FIG.
21A); however, the tabular portion 211 made of a semi-hard magnetic
material is also magnetized in a predetermined direction (e.g., in
the direction indicated in FIG. 21A), while forming a magnetic
circuit along with the pillar portions 212. Because the
magnetization direction of the tabular portion 211 has a
distribution, in effect, the magnetization is more complicated than
that illustrated in FIG. 21A; however, the magnetization as a whole
of the tabular portion 211 is exemplified, as illustrated in FIG.
21A.
In this situation, if exciting currents for the respective coils
215 are cut off, the magnetomotive force generated through the
coils 215 disappear; however, magnetization remains in the tabular
portion 211 that is made of a semi-hard magnetic material. After
the exciting currents have been cut off, as illustrated in FIG.
21B, due to the residual magnetization in the tabular portion 211,
the pillar portions 212 that are made of a soft-magnetic material
are magnetized, whereby a combined magnetic field having the same
direction as that of the magnetization in the case where the
currents are applied is formed in the open-magnetic-circuit space
as an objective space. In other words, by controlling the ratios of
the exciting currents applied to the respective coils 215, the
residual magnetization direction as a whole of the tabular portion
211 can arbitrarily be changed; as a result, the direction of a
magnetic field that remains in the open-magnetic-circuit space can
arbitrarily be changed. In magnetizing the tabular portion 211 in a
target direction, the absolute values of the exciting currents
applied to the respective coils 215 are increased, with the ratios
of the exciting currents maintained. It is preferable in terms of
control to adjust the values of the currents so that the pillar
portions 212 are not magnetically saturated.
In addition, in changing the direction of the combined magnetic
field applied to the magneto-optical element 220, it is preferable
to apply electric power that is large enough, compared with the
residual magnetization in the tabular portion 211, or to
demagnetize the tabular portion 211, by controlling the currents
supplied to the respective coils 215, and then to form a combined
magnetic field in the open-magnetic-circuit space. Accordingly, the
effect of the residual magnetization before the change of the
direction of the combined magnetic field is nullified, whereby the
direction of the combined magnetic field can appropriately be
controlled in a desired direction. Moreover, in the foregoing
magnetic circuit in FIG. 20, the direction of the combined magnetic
field applied to the magneto-optical element 220 can be controlled
so as to be oriented arbitrarily on a plane (a plane on which the
top ends of the respective pillar portions 212 are situated); by
combining with that magnetic circuit that can generate a
magnetic-field vector having a component in a direction
perpendicular to the plane, as illustrated in FIGS. 22 and 23, the
direction of the combined magnetic field applied to the
magneto-optical element 220 can be controlled so as to be oriented
arbitrarily in a 3-dimensional space.
In FIG. 22, the magnetic yoke 230 is comprised of a block-shaped
base portion 231 made of a semi-hard magnetic material and six
pillar portions 232, 233, and 234 protruding from the top and side
surfaces of the base portion 231. The base portion 231 has, e.g., a
cuboid shape; the four pillar portions 232 protrude perpendicularly
from the vicinities of the four corners of the top surface of the
base portion 231, and the respective top ends of the pillar
portions 232 are oriented to a open-magnetic-circuit space that is
surrounded by the top ends; from the one side surface of the base
portion 231, the pillar portions 234 and 233 protrude that extend
from the upper and the lower portions, respectively, of the side
surface, and whose top ends lead to a position below and a position
above the open-magnetic-circuit space, respectively. Coils 235 are
wound around the respective pillar portions 232, 233, and 234. In
contrast, in FIG. 23, a first magnetic yoke 210 having the four
pillar portions 212 as illustrated in FIG. 20 described above and a
second magnetic yoke 240 having two pillar portions 242 are
provided; arrangement is implemented in such a way that an
open-magnetic-circuit space surrounded by the top ends of the
pillar portions 212 of the first magnetic yoke 210 are sandwiched
between the top ends of the two pillar portions 242 of the second
magnetic yoke 240, from the upper and lower sides thereof. A
tabular portion 241 of the second magnetic yoke 240 is formed of a
semi-hard magnetic material, and the pillar portion 242 is formed
of a soft-magnetic material. Coils 245 are wound around the
respective pillar portions 242. In both cases, the direction of the
combined magnetic field applied to the magneto-optical element 220
can be controlled so as to be in an arbitrary direction in a
3-dimentional space, and the direction can be maintained after the
exciting currents have been cut off.
As an example of a magneto-optical device according to the present
invention, a Faraday rotator as illustrated in FIG. 20 was
produced. For the pillar portion 212, silicon steel that is a
soft-magnetic material and 3 mm by 3.3 mm in end face and 14 mm in
height was utilized. For the tabular portion 211, SUS420J2 that is
a semi-hard magnetic material and 40 mm in diameter and 0.4 mm in
thickness was utilized. For the magneto-optical element 220,
bismuth-substituted garnet single crystal of 1 mm by 1.2 mm by 0.98
mm in size was selected, and a crystal length was selected with
which, in the case of input light having a wavelength of 1550 nm,
the maximal value of the Faraday rotation angle becomes 90.degree..
In addition, the number of windings of each of the coils 215 was
set to 800; the coils 215 that are diagonally opposing each other
with respect to the magneto-optical element 220 were connected in
series with a power supply unit, and two power-supply systems were
utilized. As illustrated in FIG. 24, if, out of two pairs of coils
that are connected in series, the one pair is designated by 215a
and the other is designated by 215b, and the currents to be applied
to the respective pairs of coils are designated by current 1 and
current 2, respectively, the angle .theta. between the direction of
the combined magnetic field in the open-magnetic-circuit space and
the traveling direction of a ray, and the current values of the
respective current systems had a relationship therebetween as
represented in FIG. 25A. It can be seen that by controlling the
currents 1 and 2, a combined magnetic field can be formed in an
arbitrary direction in the open-magnetic-circuit space.
With a self-latching Faraday rotator configured as described above,
by setting the maximal values of the currents applied to the
respective coils 215a and 215b to 250 mA and controlling the ratios
and the directions of the currents, the Faraday rotation angle can
arbitrarily be adjusted over a range from -90.degree. to
+90.degree., and after the currents for the respective coils 215a
and 215b are cut off, the Faraday rotation angle in the case where
the currents have been applied can be maintained. In other words, a
self-latching Faraday rotator can be realized in which the Faraday
rotation angle after the currents for the respective coils 215a and
215b are cut off is an arbitrary angle over a range from
-90.degree. to +90.degree.. FIG. 25B represents the results of
measurement on the Faraday rotation angle in the case where the
angle .theta. between the direction of the combined magnetic field
in the open-magnetic-circuit space and the traveling direction of a
ray is over a range from -45.degree. to +135.degree.. It can be
seen that, even after the currents for the coils 215a and 215b are
cut off, the Faraday rotation angle can be maintained in the
condition that is approximately the same as that in the case where
the currents have been applied.
FIG. 26 is an explanatory view illustrating an example of a
self-latching variable optical attenuator including a
magneto-optical device according to the present invention. A
configuration is employed in which, before and after the
magneto-optical device, a light polarizer 221 and an analyzer 222
are arranged along the light path. The magneto-optical device may
be identical to that illustrated in FIG. 20; for simplicity,
corresponding members are designated by the same reference
characters.
In this situation, for the four pillar portions 212, silicon steel
that is a soft-magnetic material was utilized, and for the tabular
portion 211, SUS410J1 that is a semi-hard magnetic material was
utilized; heat treatment was applied to both materials, under
appropriate conditions. The number of windings of each of the coils
215 was set to 400; a configuration was employed in which the coils
215 that are diagonally opposing each other and wired in series are
connected with a power supply unit in such a way that the
polarities that appear at the respective top ends of the pillar
portions 212 opposing each other with respect to the
magneto-optical element 220 are reverse to each other, and the
control is implemented through two current systems. The
magneto-optical element 220 is a bismuth-substituted garnet single
crystal; the light polarizer 221 and the analyzer 222 are rutile
single crystals. The magneto-optical element 220 was 1 mm by 1.7 mm
by 0.98 mm in size, and a crystal length was selected with which,
in the case of input light having a wavelength of 1550 nm, the
maximal value of the Faraday rotation angle becomes 90.degree.. The
rutile single crystals configured a cross-Nicol arrangement, so
that, in the case where the rotation angle of the Faraday rotator
was 90.degree., the minimal attenuation value was obtained, and in
the case where the rotation angle of the Faraday rotator was
0.degree., the maximal attenuation value was obtained.
According to the self-latching variable optical attenuator
configured as described above, by setting the maximal values of the
currents applied to the respective coils 215 to 250 mA and
controlling the ratios and the directions of the currents, the
Faraday rotation angle can be converted into a desired angle, and,
as represented in FIG. 27, the value of light attenuation can be
adjusted to an arbitrary value over a range from 1 dB to 22 dB.
Besides, after the currents for the respective coils 215 are cut
off, the Faraday rotation angle is maintained in the condition as
the currents have been applied; therefore, the value of light
attenuation can also be maintained in the condition as the currents
have been applied. In other words, a self-latching variable optical
attenuator can be realized in which, the value of light attenuation
obtained after the currents for the respective coils 215 are cut
off is arbitrary over a range from 1 dB to 22 dB. The device is 47
mm by 47 mm by 25 mm in size.
FIG. 28 is an explanatory view illustrating an example of a
self-latching variable optical splitter including a magneto-optical
device according to the present invention. The optical splitter is
configures in such a way that, from the input-port side to the
output-port side, a rutile single crystal plate 251a, a
magneto-optical device, 1/2-wavelength plates 252a and 252b, a
rutile single crystal plate 251b, 1/2-wavelength plates 252c and
252d, and a rutile single crystal plate 251c are arranged in that
order. The magneto-optical device is a self-latching Faraday
rotator, among self-latching Faraday rotators as illustrated in
FIG. 20, that utilizes the magneto-optical element 220 that
provides a Faraday rotation angle of 90.degree.. In FIG. 28, the
magnetic-circuit portion of the Faraday rotator is omitted. In
addition, the directions of the crystal axes of the three rutile
single crystal plates 251a, 251b, and 251c are indicated by the
arrows in FIG. 28; the rutile single crystal plate 251a has a
function of splitting a ray consisting of subrays that are in the
same light path and whose polarization directions are perpendicular
to each other; the rutile single crystal plate 251b has a function
of controlling a light path in accordance with a polarization
direction; and the rutile single crystal plate 251c has a function
of synthesizing rays that are in different light paths and whose
polarization directions are perpendicular to each other. The four
1/2-wavelength plates 252a, 252b, 252c, and 252d each have a
function of rotating by a predetermined angle the polarization
direction of a ray.
In the self-latching variable optical splitter configured as
described above, an incident ray to the input port are separated
into two subrays, and the subrays are emitted through output port 1
and output port 2; the separation ratio can be controlled through
the values of currents supplied to the Faraday rotator. The
separation ratio excluding the insertion loss can be varied over a
range from 0:100 to 100:0; obtained crosstalk values were above 42
dB. After the currents for the Faraday rotator are cut off, the
Faraday rotation angle is maintained in the condition in the case
where the currents have been applied; therefore, as is the case
where the currents have been applied, an arbitrary separation ratio
can be maintained.
By combining the magneto-optical device illustrated in FIG. 20, a
rutile single crystal plate, and a 1/2-wavelength plate, it is
possible to configure a self-latching variable optical switch.
According to the self-latching variable optical switch, by
controlling the ratios and directions of the currents applied to
the respective coils 215, the light path can be switched so as to
be in a desired condition, and after the currents for the coils 215
are cut off, the condition can be maintained. Moreover, in
Embodiment 3, the pillar portion is formed of a soft-magnetic
material, and the tabular portion is formed of a semi-hard magnetic
material; conversely, it is possible that the pillar portion is
formed of a semi-hard magnetic material, and the tabular portion is
formed of a soft-magnetic material. In this case as well, after the
exciting currents are cut off, the direction of the combined
magnetic field that remains in the open-magnetic-circuit space can
be controlled so as to be in an arbitrary direction, whereby the
Faraday rotation angle can be maintained so as to be in an
arbitrary condition.
INDUSTRIAL APPLICABILITY
A magneto-optical device according to the present invention can be
utilized in a variable optical attenuator, a variable optical
splitter, or the Faraday rotator in a optical switch. By generating
variable magnetic fields by utilizing electromagnets only, without
utilizing any fixed magnetic field of a permanent magnet, the
magnetization direction can instantaneously be changed, whereby the
magneto-optical device can be downsized and its operation is
speeded up. Since, regardless of the direction of the combined
magnetic field, no large magnetic field is required, the coils can
be downsized, whereby the driving voltage can also be reduced.
Since a pair of coils diagonally opposing each other with respect
to a magneto-optical element is configured and the coils in the
pair are wired in parallel or in series and driven by a common
power supply unit, the drive of the coils can efficiently be
implemented with a small number of power supply units, whereby the
peripheral circuitry can be simplified.
Moreover, by forming with a semi-hard magnetic material the tabular
portion or the pillar portion of the magnetic yoke, even after the
exciting currents are cut off, the direction of the combined
magnetic field that remains in the open-magnetic-circuit space can
be controlled so as to be in an arbitrary direction, whereby the
Faraday rotation angle can be maintained so as to be in an
arbitrary condition.
* * * * *